The manipulation of input and output light signals to and from optical fiber transmission lines generally requires that the signals be processed in some fashion, examples of which might include amplification, power splitting or the addition and/or dropping of signals. With the persistent trend towards miniaturization and integration, the optical circuits which best serve these processing functions are more and more being integrated on optical chips as a single module. The resulting optical circuits, which carry channel waveguides as their fundamental light-guiding elements, are generally referred to as planar lightwave circuits or PLCs. Current planar waveguide technology typically prepares a PLC by depositing a sequence of three glass films (lower cladding, core and upper cladding) on a rigid planar substrate and utilizing photolithography to pattern the required waveguide and component designs into the core layer. The refractive index of the core composition is chosen to be larger than those of the cladding layers to ensure good optical confinement within the core waveguides.
In optical networks it is necessary to monitor the level of the propagating light signal at several points in the system. As more and more functions are integrated in photonic lightwave circuits, integrated tapping devices, tapping a small fraction of the light, are needed to monitor the signal power. Although Y-branching circuits with equal power division are fundamental building blocks for optical signal processing devices, any asymmetric adaptation of this form with a branching angle large enough to achieve compactness is unable to tap out a sufficient power fraction for many applications. An optical tap representing the current art typically comprises a pair of side-by-side channel waveguides, or directional couplers, in which structure the light signal in one waveguide is evanescently coupled to the other waveguide. The fraction of light tapped-out (tap efficiency) is controlled by the distance between the two waveguides and the by the length along which they couple. Unfortunately, the optical response of a directional coupler in general depends strongly both on the polarization and wavelength of the light signal to be tapped, a characteristic that is undesirable for a versatile optical network component.
Two types of integrated optical taps have been proposed that are both polarization independent and wavelength insensitive. FIG. 1 illustrates the optical tap proposed by Henry et al. (U.S. Pat. No. 5,539,850). The invention comprises two directional couplers 101 and 102 in series in which the second coupler 102 compensates for the wavelength and polarization dependencies of the first coupler 101. The light signal is input at port 103 and most of it exits at port 104, while a small amount is tapped off to port 105. This design, however, has several disadvantages. For example, the size of such a coupler cascade is large (typically a few mms), and it also possesses an inherent loss mechanism due to light dumped from port 106 of the device. A different design for a compact integrated tap has been disclosed by Adar et al. (U.S. Pat. No. 5,276,746) and is illustrated in FIG. 2. It utilizes the guide-interaction properties of an X waveguide crossing to tap out a low level (−20 dB to −60 dB) signal. Light signal is input in port 201, passes through the X-crossing 202 and most of the light exits at port 203 while a small amount of power is tapped off to port 204. Due to symmetry, light can also be input at port 205, in which case most of the light exits at port 204 and a small amount will be tapped off to port 203. This design is also polarization independent, but the signal power fraction that can be tapped out using a crossing angle large enough to achieve device compactness is (as is the case for the Y-junction) insufficient for many applications. Moreover, the low index contrast between the cladding and the waveguide core materials, combined with the large crossing angle (>10 degrees), results in a low tap efficiency.
The mechanism of light transfer between the arms of a pair of intersecting waveguides is, at least for small crossing angles, qualitatively similar to that of a directional (i.e. evanescent) coupler with variable inter-guide separation. At the X-branch geometric crossover between two guides A and B, the incoming optical field (say in branch A) can be pictured as the sum of equal-amplitude symmetric and antisymmetric component fields in the two incoming branches. Where they begin to interact on approach to the junction, these two component fields will in general develop different velocities (and possibly different rates of attenuation). In the output branches the two fields (minus their radiative and absorption losses) can be recombined taking their relative phase shifts into account. A phase shift of π/2, for example, would cause light to be wholly transferred from A to B. More generally the degree of transfer from A to B at any point of the crossover will depend on the phase difference accumulated to that point and, for small crossing angles (with a large interaction length) the light power may alternate back and forth several times before emerging from the crossing. The final degree of transfer therefore depends on the total phase difference accumulated over the entire crossover region. In this simple picture (see, for example, Bergmann et al., Applied Optics 23, 3000-3003 (1984)) the fractional power transferred between the waveguides is approximately periodic in the reciprocal of the crossing angle θ with a period that depends sensitively on the magnitude of the guide refractive index contrast Δn=n(core)−n(cladding) in the crossing regime. As a result of this sensitivity, most of the current applications of waveguide crossing structures are in the field of optical switches, and are based on the use of an external (electro-optic, magneto-optic, acousto-optic or thermo-optic) stimulus to modulate Δn in the region of the crossing.
At crossing angles larger than a degree or two the periodicity in 1/θ ceases and the power-fraction transferred from the signal waveguide to the tap waveguide decreases rapidly to extremely small values at larger crossing angles. Unfortunately, this is the angular region of relevance for the formation of compact waveguide-crossing taps.